refscale: Disable verbose torture-test output

Given large numbers of threads, the quantity of torture-test output is
sufficient to sometimes result in RCU CPU stall warnings.  The probability
of these stall warnings was greatly reduced by batching the output,
but the warnings were not eliminated.  However, the actual test only
depends on console output that is printed even when refscale.verbose=0.
This commit therefore causes this test to run with refscale.verbose=0.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>